Chromosomes must be equally segregated during cell division. Eukaryotes use microtubules to control the positioning and movement of chromosomes. In addition to kinetochores and centrosomes, the chromatin that makes up the chromosome arms plays important roles in organizing spindle microtubules. The long-term goal of this proposal is to understand how chromosomal factors influence the local environment and coordinate chromosome segregation. To approach this goal, we used expression screening to identify new metaphase chromosome-binding proteins, and discovered Dasra A and Dasra B, two new components of the vertebrate chromosomal passenger complex, which contains the kinase Aurora B. This complex plays roles in the spindle checkpoint and cytokinesis, and it has been proposed to regulate microtubule dynamics. Whereas Dasra B is conserved among vertebrates, Dasra A homologs can be found in fishes, frogs and chicken, but not in mammals. One of the characteristics distinguishing Dasra A-containing vertebrates from mammals is the rapid early embryonic cell divisions during which the checkpoint controls are suppressed, suggesting that Dasra A may be specialized to regulate spindle assembly in these rapid cell division cycles. Using Xenopus egg extracts, we propose to: 1) Determine the full constituents of the chromosomal passenger complex. 2) Distinguish the functions of Dasra A and Dasra B in chromatin-induced spindle assembly, in activation/maintenance of the spindle assembly checkpoint, and in the regulation of the kinase activity of Aurora B. 3) Examine how the chromosomal passenger complex is regulated on chromosomes. Do chromosomes biochemically activate the chromosomal passenger complex, or are chromosomes simply used for increasing local concentration of the complex? The findings of this proposal will help us understand how normal cells segregate their chromosomes accurately and how this process fails in cancer cells and in birth defects.